PROJECT SUMMARY: Immune checkpoint inhibition of the PD-1/PD-L1 pathway has demonstrated dramatic and durable clinical benefit for patients in many cancer types and is increasingly being incorporated into standard of care treatment regimens. However, in most tumor types, this strategy is effective only for a minority of patients. As such, there is an urgent need for novel therapeutic approaches to potentiate immune checkpoint blockade and increase the proportion of patients who benefit from it. We recently used in vivo CRISPR screening in transplantable mouse tumor models treated with antibodies targeting PD-1 to identify the RNA editing enzyme, ADAR1, as a novel target for combination immunotherapy. In a manuscript published in Nature, we showed that loss of ADAR1 can overcome two of the most common and challenging mechanisms of resistance to immunotherapy: a lack of infiltration of anti-tumor immune cells in the tumor microenvironment and the loss of antigen presentation by the class I major histocompatibility complex on tumor cells. However, the mechanisms by which loss of ADAR1 achieves these therapeutic goals are unclear. In Aim 1 and Aim 2 of this grant, I propose to define these mechanisms and to elaborate general principles by which resistance to immunotherapy can be overcome. More broadly, the identification of ADAR1 as a target for improving responses to immunotherapy suggests that targeting other enzymes that are associated with a similar auto- inflammatory phenotype in humans could similarly benefit patients treated with immune checkpoint blockade. I propose to test this hypothesis in Aim 3 of this grant. If successful, these results will provide a mechanistic paradigm for overcoming resistance to immunotherapy and identify novel drug targets that trigger a similar mechanism. I am currently a Clinical Fellow in Medical Oncology in the Dana-Farber/Partners CancerCare Hematology and Oncology Fellowship Program. Over 85% of my time is devoted to my ongoing research under the mentorship of Dr. Matthew Meyerson (DFCI, Broad Institute) and Dr. Arlene Sharpe (Harvard Medical School). The remainder of my time is devoted to clinical practice and training at the Dana-Farber Cancer Institute, primarily in thoracic and gynecologic medical oncology. My goal is to successfully transition from senior fellow to research instructor and ultimately to an independent investigator in a tenure-track position. I am applying for the K08 Mentored Clinical Scientist Research Career Development Award to provide the necessary training and funding to achieve this goal. Under the mentorship of Drs. Meyerson and Sharpe, and the guidance of my advisory committee (Drs. Golub, Freeman, Hur and Barbie), I will access the necessary resources and training to develop a successful, independent research program over the funding period of the award.